A stratified charge fed inside an internal-combustion engine is a charge comprising at least one portion in which the fuel is at its "ignition richness". The "ignition richness" of a homogeneous air-fuel mixture at a given temperature is the fuel proportion allowing a flame to be propagated within the mixture, either by means of external energy (a spark plug, for example) or not.
Extensive research has already been made in the field of stratified-charge engines which use liquid or gaseous hydrocarbons and numerous solutions have been proposed although effective stratification of the charge has not yet been obtained. Effective stratification is obtained when a core having the proper ignition richness is formed about the point of ignition, the energy generated through combustion of this core being sufficient to ensure flame propagation in the remaining charge.
As far as liquid hydrocarbons are concerned, it is known that an air-fuel ratio of about 15/1 is the optimum ratio for obtaining ignition upon contact with a spark such as that generated by a conventional ignition system. It is also known that at a ratio of above 18/1 it becomes difficult to ignite this mixture other than by use of a live flame.
The use of a conventional air-fuel mixture requires a long preparation time due to three stages absolutely necessary for obtaining the ignition of the mixture, i.e.:
Vaporization of the liquid fuel (however, it may be mixed in a fully atomized condition in some cases); PA1 Homogenization of the mixture; PA1 Temperature increase of the mixture (in such a way as to attain a temperature level sufficient to ensure flame propagation).
Since the preparation time is long with respect to the times imposed by the speed of rotation of the engines, a portion of charge having a richness level close to that of a stoichiometric mixture has to be added to a portion of the charge having a much lower richness level in a sufficiently short period to allow the three previously mentioned stages to take place. Under these conditions, it is very difficult to prevent the two charge portions --which must, of course, be in contact with one another in order that complete charge combustion may occur from mixing together to some extent. The mixture of the two charge portions, thus necessarily formed, decreases the richness of the rich portion of the charge and increases the richness of the lean portion, which is just the opposite of the stratification sought.
Moreover, it is known that difficulties due to poor vaporization and homogenization owing to lack of time arise in internal-combustion engines which are fed by direct injection into the cylinders.
Poor vaporization and homogenization are also responsible for major difficulties in "Diesel" engines which require injection to be carried out at pressures higher than 150 kg/cm.sup.2 in order to atomize the fuel. Full-load conditions can never be fully attained for lack of homogenization.